Combustion system

ABSTRACT

A combustion system for a heat generator, the burner  1  being connected to a combustion chamber  6  by means of an outlet  10 , wherein the outlet  10  comprises a multiply stepped transional structure  11  in the direction of flow of fluid  3  in the burner  1  so as to create turbulence in the fluid flow.

The present invention relates to a combustion system for generating ahot gas, and in particular to a premix burner connected to a combustionchamber.

Many premix burners rely on swirling to produce efficient mixing ofreactants. However, interaction between the complex flow patterns withinthe swirling fluid and acoustic resonant modes in the combustion chambercan lead to undesired thermoacoustic pulsations or vibrations. Thesepulsations are associated with coherent vortical flows in the combustionchamber. The vortical flows introduce periodicity into the mixingprocess, which may lead to periodic heat release and resonant couplingwith the combustor acoustic resonant modes. Vortical mixing of thereactants also tends to be limited to large scale mixing with the resultthat mixing in regions between vortices in the vortical flow tends to bepoor.

Thermoacoustic vibrations are problematic in combustion processes, sincethey can lead to high-amplitude pressure fluctuations, as well as to alimitation in the operating range of the burner in question and toincreased emissions from the burner. Many combustion chambers do notpossess adequate acoustic damping to account for such thermoacousticvibrations.

In conventional combustion chambers, the cooling air flowing into thecombustion chamber acts to dampen noise and therefore contributes to thedamping of thermoacoustic vibrations. However, in modern gas turbines,an increasing proportion of the cooling air is passed through the burneritself in order to achieve low emissions. The cooling air flow withinthe combustion chamber is thus reduced, resulting in reduced damping ofthe thermoacoustic vibrations in the chamber.

Another method of damping is the coupling of Helmholtz dampers in thecombustion chamber, preferably in the region of the combustion chamberdome or in the region of the cold air supply. However, such dampersrequire a considerable amount of space in order to allow them to beaccommodated in the combustion chamber. Since modern combustion chamberstend to be relatively compact, it is usually impossible to incorporateHelmholtz dampers in the combustion chamber without substantialre-design of the chamber.

A further method of controlling thermoacoustic vibrations involvesactive acoustic excitation. In this process, a shear layer which formsin the outlet region of the burner is acoustically excited. A suitablephase lag between the thermoacoustic vibrations and the excitationvibrations makes it possible to achieve damping of the combustionchamber due to the superimposition of the vibrations and the excitation.However, a considerable amount of energy is expended in generating suchacoustic excitation.

A further means of providing damping in the combustion chamber is tomodulate the fuel mass flow in the burner. Fuel is injected into theburner with a phase shift relative to measured signals in the combustionchamber so that additional heat is released at a minimum pressure Thisreduces the amplitude of the thermoacoustic vibrations. However, thistechnique also leads to high emissions due to the increased fuel.

A further alternative is to inject air into the burner via nozzles todisturb and break up the vortical flow. However, the required additionalpipes and plumbing complicates the design of the combustor. Furthermore,the required additional air flow reduces the overall efficiency.

In a similar technique, the vortical flow is broken up by baffles whichare located inside the burner in order to disturb the vortical flow.However, the inclusion of such baffles increases the constructionaloutlay of the burner, which is disadvantageous.

An object of the present invention is to provide a combustion system inwhich the above disadvantages are overcome.

The invention provides a combustion system for a heat generator,comprising a premix burner and a combustion chamber, the premix burnerbeing connected to the combustion chamber by means of an outlet, whereinthe outlet comprises a multiply stepped transitional structure in thedirection of flow of fluid so as to create turbulence in the fluid flow.

In contrast to the sharp-edged transition between the premix burner andthe combustion chamber the combustion system designed according to theinvention has a gradual transition between the premix-burner and thecombustion chamber, said transition having a segmented line-up ofrectilinearly designed side wall portions forming a multiply steppedtransitional structure. The term “multiply stepped transition” isintended to mean basically any transitional geometry which widens insteps the flow cross section within the premix burner, which isdimensioned smaller than that within the combustion chamber,successively to the combustion chamber cross section.

In a preferred embodiment of the invention, the transitional structurecomprises three to five steps, and preferably four.

By a gradual transition being provided between the premix burner and thecombustion chamber, the widening of the fuel/air mixture entering thecombustion chamber is increased considerably, the result of this being,even in the case of a gradual transition, that a marginal flow havingcross vortices is formed, which, however, impinges onto the combustionchamber wall at a reapplication point which is very much nearer in thedirection of the premix burner than in the case of a sharp-steppedtransition. This has an advantageous effect on the combustion process intwo respects. Thus, on the one hand, the marginal flow having crossvortices is reduced, and therefore the intensity and number of the crossvortices formed are also reduced, with the result that the combustionchamber pulsation generated by thermoacoustic vibrations can bedecisively damped. On the other hand, by virtue of the markedly greaterwidening of the fuel/air mixture propagated within the combustionchamber, the dead space caused by shading-off effects is reduced to aminimum, with the result that virtually the entire combustion chambervolume is available for the combustion of the fuel/air mixture andensures complete combustion of the fuel.

The invention will now be described in detail with reference to theaccompanying drawings, in which:

FIG. 1 is a cross-sectional view of a burner according to the inventionattached to a combustion chamber;

FIGS. 2 a and 2 b are graphs showing the effect of the invention onpressure fluctuations.

In FIG. 1, a heat generator has a burner 1 with a swirl generator 2. Theswirl generator 2 generates a swirl 3 with an axial flow componentfacing toward a downstream burner outlet 4. Mixing takes place in anarea 5 of the generator 2, so as to ensure adequate mixing of fuel andcombustion air. The axial flow cross-section of the area 5 widens in thedirection of the outlet 4; this configuration facilitates attainment ofa constant swirl 3 in the area 5 with an increasing combustion air massflow in the direction of the longitudinal axis B of the burner 1. Thegenerator 2 comprises two hollow partial cones (not shown) arrangedoffset to one another. The offset of the respective centre axes of thepartial conical bodies creates two tangential air channels 6. Acombustion air flow 7 flows, with a relatively high tangential velocitycomponent, through the two tangential channels 6 into the area 5, thusgenerating the swirl 3. Fuel is introduced into the burner 1 via a fuelinlet 8 in the form of a nozzle.

The burner 1 is attached to a combustion chamber 9 via an outlet 10through which the swirl 3 passes. The swirl 3 contains vortical flow,which causes flow instabilities including thermoacoustic vibrationswhich result in low performance of the combustion chamber.

The outlet 10 is provided with a series of steps 11, 11 a and 11 b. Thesteps 11, 11 a and 11 b induce multiple inflection points into the swirl3 as a result of the sudden change of velocity of the flow at the steps11, 11 a and 11 b. Multiple sources of turbulence are thus formed. Thisincreased turbulence serves to break up the existing vortical flow inthe swirl 3, thus stabilising the flow. As a result the performance ofthe combustion chamber 6 is improved. Furthermore, the increasedturbulence results in better small scale mixing. It should be noted,however, that emissions are not noticeably increased as a result of theincreased turbulence.

The preferred range of the ratio of the length to the height of thesteps 11, 11 a, 11 b is 1:1-7:1, but can be as large as 10:1. The numberof steps depends on the expansion ration at the outlet 10, on there-attachment length, and the selected length to height ratio. Thenumber of steps is usually between three and five. However, one singlestep can be effective. This is particularly so, if the step height isthe same as the amplitude of the dominant vortices.

FIG. 2 a shows the effect of the burner according to the invention onpressure fluctuations according to variation in Lambda number. Line 12is effectively a baseline, i.e. it represents a burner which has notbeen modified in any way. Line 13 represents a burner having steps 11,11 a and 11 b with a length to height ratio of 1:1. Line 14 represents anozzle with extended steps, i.e. steps extended beyond a recirculationzone. This configuration can lead, however, to a destabilisation incombustion.

FIG. 2 b shows the effect of the burner according to the invention onpressure fluctuations according to variation in power. Line 12 a iseffectively a baseline, i.e. it represents a burner which has not beenmodified in any way. Line 13 a represents a burner having steps 11, 11 aand 11 b. Line 14 a represents a burner with extended steps.

It will be appreciated that variations of the embodiment described aboveare possible. Alternative configurations of pre-mix burners arewell-known to persons skilled in the art. Similarly, it would bepossible to replace the conical swirl generator 2 with a cylindricalswirl generator. It is also known to arrange a displacement body,tapering towards the outlet 10, inside the swirl generator; this couldprovide a further alternative embodiment of the invention.

The number and depth of the steps could also be varied.

1. A combustion system for a heat generator, comprising: a premixburner, and outlet, and a combustion chamber, the premix burner beingconnected to the combustion chamber by the outlet; wherein the outletcomprises a multiply-stepped transitional structure in the direction offlow of fluid so as to create turbulence in the fluid flow.
 2. Acombustion system as claimed in claim 1, wherein the transitionalstructure comprises three to five steps.
 3. A combustion system asclaimed in claim 2, wherein the transitional structure comprises foursteps.
 4. A combustion system as claimed in claim 1, wherein the lengthto height ratio of the steps is from 1:1 to 10:1.
 5. A combustion systemas claimed in claim 4, wherein the length to height ratio of the stepsis from 1:1 to 7:1.
 6. A combustion system as claimed in claim 1,wherein the outlet is in the form of comprises a nozzle.
 7. (Canceled)